1. Field of the Invention
The present invention relates to an apparatus for directly transporting a tape (e.g., magnetic tape) from one reel to another reel and, more particularly, to a tape transport method and an apparatus suitable to transport a tape at a high speed and with a high degree of accuracy.
2. Description of the Prior Art
A conventional tape transport apparatus is disclosed in, for example, U.S. Pat. No. 4,125,881 issued on Nov. 14, 1978 and entitled "Tape motion control for real-to-reel drive". According to the apparatus disclosed in this U.S. Patent, the drive currents of respective motors to drive a supply reel and a take-up reel are calculated on the basis of the radius of tape wound around each reel and the drive currents are supplied to the respective motors, thereby allowing the speed and tension of the tape in a magnetic head portion to be maintained constant. On the other hand, to realize stable operation of the apparatus, the actual reel drive system needs to be the inherent system according to the design. This is because when the drive currents to be supplied to the motors for driving two reels are calculated, various kinds of set constants for use in the calculations, and the other set values and the like are determined on the basis of the set constants regarding the design reel drive system on the assumption that the actual drive system has been manufactured and can operate in accordance with the inherent design. However, in general, the actual reel drive system unfortunately differs from the system as designed because of variations in dimensions, characteristics, and performances of respective parts or due to various errors in assembly and the like. In particular, the tape transport apparatus has two reel drive systems to respectively drive two reels, so that the variations in performance, characteristics, and the like of two reel drive systems will become a serious obstacle to the stable tape transport between two reels. Such variations cause a fluctuation in tension (tape tension) which will be applied to the tape upon transport of the tape or makes it difficult to stably transport the tape at a high speed, and the like. Even if the quality is carefully managed, those variations in performance and characteristics will certainly occur. In general, to control the tape tension to a predetermined value, the tape tension is detected and compared with a set tension to obtain the tension error, and the drive currents to be supplied to the reel drive motors are corrected so as to eliminate the tension error in a feedback manner. The tape transport apparatus disclosed in U.S. Pat. No. 4,125,881 also controls the tape tension in a feedback control manner as mentioned above. However, it is difficult to maintain the tape tension to a predetermined value by such a feedback control. In particular, for the period when the reel is accelerated from the stop state to the state of a predetermined speed or for the period when the reel is stopped from the predetermined speed, a large tension variation occurs by the variations in performance and characteristics of two reel drive systems. Thus, the satisfactory tension control cannot be accomplished by the foregoing feedback control because of the operation time lag of the control system.